This invention relates to a spindle of a machine tool, and more particularly a spindle capable of not using as far as possible metal materials but decreasing the weight of the spindle and the adverse effect of linear thermal expansion.
As the material of the spindle utilized for a machining center, a boring machine, and a milling machine has generally been used steel or alloy steels having a large weight because steel and alloy steels have a high Young's modulus, can harden their surfaces, easy to machine and are not expensive.
The linear expansion coefficient of steel lies in a range of 11.about.16.times.10.sup.-6 .degree. C.sup.-1. Thus, in a spindle having a length of 2 m, when the temperature varies by 10.degree. C., the length of the spindle would vary in a range of 2.2.about.3.2.times.10.sup.-1 mm. For example, during the operation of a boring machine, where the temperature of the spindle rises by 10.degree. C. from the start to the end of the machining, the cut depth would vary by about 0.3 mm, thus failing to obtain machined products having high dimensional accuracies. Moreover, since steel has a large specific gravity and the weight of the spindle made of steel increases, thus resulting in a large deformation due to centrifugal force at the time of high speed operation as well as in the nonuniformity of the dimensions of the machined products. Furthermore, as the vibration attenuation of steel is small, a spindle made of steel is not suitable for high speed rotation. Furthermore, where the spindle weight is large it takes longer time for building up to a high speed which requires provision of a large power source.
As materials having a small weight and a high rigidity have been known such fiber reinforced composite materials as carbon fiber reinforced plastic material (CFRP), and such materials are now being used in many industrial fields. However, the fields capable of using such materials have been limited. Usually, these materials have been used for producing products for use in public welfare and members for use in aeroplanes and it has been extremely rare to use these materials as structural members of machines. As special examples, rollers are made of these materials as disclosed in Japanese Laid Open Patent Specification No. 194197/1986 and Japanese Laid Open Utility Model Specification No. 69812/1988. These rollers were developed to have specific characteristics not found in the prior art metal rollers. More particularly, the composite materials described above, particularly CFRP was used to manufacture rollers by noting its specific characteristics, that is an extremely light weight and a high rigidity. However, these rollers are constructed to be supported at both ends so that they are used in paper making machines, printing machines, synthetic resin film manufacturing machines, etc. These rollers have been used in these machines because it is possible to minimize as far as possible the flexure at the central portion of the roller, and can accurately follow the speed variation at the time of quickly stopping the roller and varying the speed of the roller. These fiber reinforced composite materials do not have sufficient mechanical strength capable of withstanding mechanically severe conditions. For this reason, although rollers disclosed in Japanese Laid Open specifications described above are applicable to certain industrial fields, they cannot be used as structural members subjected to severe operating conditions, especially as a spindle of a machine tool.
When cutting or grinding metal material or hard and brittle material such as ceramics, the spindle of a machine tool is subjected to a heavy load in all directions. Moreover as the spindle has a cantilever construction, steel or alloy steels have been exclusively used for the spindle for the reason that these materials have high degrees of elasticity and rigidity, and can readily harden their surfaces. It has been recognized that it is impossible to practically use fiber reinforced plastic material having low hardness and low wear resistant property, although it is light and has a high rigidity.
As disclosed in Japanese Laid Open Patent Specification No. 96311/1988, use of carbon/carbon (C/C) composite material similar to the fiber reinforced plastic material for the manufacture of the spindle of a machine tool has been proposed.
The C/C composite material has been used for many years for preparing machine parts, parts of a nuclear reactor and space machines (see "Carbon Fiber" published on Nov. 1, 1955, authors: Otani and Kimura, by Kindai Henshusha, Japan). Although the inventors of this invention have also tried to apply the C/C composite material to the spindle of a machine tool it was found that this material is not suitable for such use. Consequently, the inventors have determined to use the fiber reinforced plastic material.
The C/C composite material can be obtained by pyrolyzing the fiber reinforced plastic composite material at a high temperature for carbonizing plastic material used as a matrix. However, the resulting C/C composite material has inferior physical characteristics, so that it is necessary to subject the C/C composite material to an additional processing for improving its physical characteristics. But to obtain desired characteristics it is necessary to use many steps of working, resulting in cost up.
The spindle made of steel has a poor dimensional stability due to a high linear expansion coefficient inherent to steel. Although it is possible to reduce the linear expansion coefficient by using metals other than steel for example, an invar alloy having a smallest linear expansion coefficient of 1.about.3.times.10.sup.-6 .degree. C.sup.-1, invar is difficult to machine. The invar alloy cannot be used as the main material of a spindle because of its high cost and low hardness.
Since a spindle made of steel has a large weight, the spindle deforms greatly due to the centrifugal force created at the time of high speed running and since the attenuation characteristic of spindle vibration is poor, the quality of machined products and the machining accuracy are poor. Moreover, since the spindle made of steel has a large moment of inertia, it is difficult to increase the machining speed. Moreover the heavy steel spindle requires a large driving power and generates a large quantity of heat which decreases the machining accuracy.
In order to obtain a spindle using the C/C composite material it is necessary to use a number of process steps which increases the manufacturing cost.
In Japanese Laid Open Utility Model Specification No. 165101/1985 is disclosed a spindle of a machine tool utilizing a hollow cylinder shaped fiber reinforced plastic material. In this reference, the hollow cylinder is interposed between a tapered shank driven by a driving source and a tool holder made of steel and adapted to hold a tool. With this construction, the hollow cylinder is not used to directly receive the shank of the tool so that the advantage of the fiber reinforced composite material is not fully utilized.